Frequency modulation system



4 Sheets-Sheet 1' Oct. 13, 1942.-

V Filed Oct. 17, .1940

a x l T SIG/VAL SOURCE ourpur Q SIG/VAL INVENTOR SOURCE GEORGE L. gnu/wV Y B M ATTORNEY I Oct. 13, 1942. G. 1.; USSELMAN FREQUENCY MODULAT IGNSYSTEM Filed Oct. 17, 1940 4 Sheets-Sheet 2 our ur I El lGW/JL LINVENTOR GEORGE ,L. l/SSELMAN 46 SOURCE ATTORNEY e. USSELMAN 2,298,438

FREQUENCY MODULATION SYS TEM Oct. 13, 1942.

Filed Oct. 17, 1940 4 Sheets-Sheet 3 g ourpur f EI FE;

SI/GAIAL SOURCE INVENTOR GEORGE L. USSELMAN ATTORNEY Oct. 13, 1942. V EM 2,298,438

FREQUENCY MODULATION SYSTEM Filed Oct. 17, 1940 4 Sheets-S eet 4 OUTPUTAAA SIG/VAL SOURCE INVENTOR GEORGE LUSSELMAN I grids grounded for radiofrequency.

Patented Get. 13;, 1942 FREQUENCY Monuaa'rrorr srs'rmu George L.Ussclman, Port Jefferson, N. 12, assignor to Radio Corporation ofAmerica, a. corporation of Delaware Application October 17, mo, SerialNo. 361,506

6 Claims.

This application concerns a new and improved method of and means forproducing wave energy the frequency of which is modulated in accordancewith signals.

The crystal oscillator circuit used in this frequency modulator is ofthe aperiodic type, except for the piezo-electric crystal, having theanode These fea tures contribute considerably to the improved operationof this circuit. The principles involved of a separate condenser 6 asshown in Figs-1.

j 2, 3 and 4. If desirable, a tube having only two in my presentdisclosure are similar in many respects to those involved in my UnitedStates application #338,837 filed June 5, 1940 and in my United Statesapplication #338,838 filed June 5,

, for deriving a voltage from said circuit, suppl ing the same to anamplifier tube the gain of which is controlled in accordance withcontrol potentials with means for supplying the amplified voltage backto the generator circuits in phase displaced relation with respect tothe voltgrids may be used in place of VI. In this case, grid G2 as shownin Fig. 7 serves in place of the present grids G2 and G3. Both thesecond and third grids in Figs. 1, 2, 3 and 4 are supplied with positivepotentials through resistors 3 and 3' from a source not shown by meansof lead 8. Resistors 3 and 3' act as a periodic smoothing impedances-tokeep radio frequency out of lead 8. The anode III of tube Vi is suppliedwith positive current and potential through tank circuit CI, Li, from a.power source not shown. This'source may be the same as the source whichsupplies voltage to lead 8 or it may be a separate source. The outputcircuit I2 for the frequency modulator is coupled through a couplingcondenser CC to the tank coil Ll in Figs. 1 to 4,- inclusive. The outputcoupling may be inductive, as shown in Figs. 5 and 6. The control gridGI of the oscillator tube Vi is connected to one of the crystalelectrodes, the other crystal electrode being connected to the cathodeof tube VI age generated therein to produce a controllable cations ofthe corresponding couplings of Fig. 1.

In Fig. "I, the excitation voltage for the modulator tube is derivedfrom the output circuit of through circuits described in detail later tothereby be included in the oscillator circuits. The potentiometer gridleak resistor R! is connected between the control grid G! of tube VI andground. The cathode l8 of tube VI is connected to a movable tap B onresistor R! The modulator circuits comprising reactances L2, C2, C3,tube V2, etc., are substantially alike except for minordifi'erences asstated below. In all of the modifications shown in Figs. 1 to 6,inclusive, the cathode 20 of tube V2 is grounded. The control grid 24 oftube V2 is excited by high frequency oscillations because it isconnected to a a tap on resistor RI through a blocking conthe oscillatorandthe oscillator, as well as the modulator, are controlled by thecontrolling po- The thirdgrid G3 maybe grounded directly as shown inFigs. 5 and 6 or it may be biassed through a resistance 3 and bydenser28 as shown. The high frequency 6501]- lations are amplified in V2 andreturned via the crystal X in shunt or in series to the grid G of tubeVl so that the phase of excitation of grid G! is made up of two voltagescomponents, one due to regeneration in Vi and the other fed back throughV2. The latter varies in phase as will be described later and thisproduces phase and frequency modulation of the generated wave. Fig. 1shows a phase shifting reactance P connected in this modulator gridradio-frequency excitation circuit. The modification of Fig. 7 will betaken up separately because of the differences between it and thecircuits of the prior figures.

The two voltage components on grid GI discussed abov are displaced inphase in the unmodulated condition and during modulation this passed toground for radio frequency by mean phase displacement is varied as willbe described more in detail hereinafter. The phase displacement may beintroduced by a phase shifter P or by proper tuning of the circuits L2,02, as will be described later.

A resistor R2 is connected to the control grid 24 of tube V2.v The otherend of resistance R2 is -by-pas sed to ground for radio frequency bymeans of a condenser 30 and it is also connected to one terminal of thesecondary winding of transformer lead 8.

The anode 40 of modulator tube V2 is tapped to a point on L2 and issupplied with positive current and potential through the coil L2 of tankcircuit C2, "L2, in Figs. 1, 3. 4, and 6. In Fig. 2,

the anode 40 of V2 is tapped to a point on an inductance L3 in serieswith coil L2 and receives its positive current and potential supplythrough these inductances. In Fig. 4, a resistance R3 is included in thedirect-current circuit of anode 40 of tube V2. In Fig. 5, the anodealternatingcurrent circuit of tube V2 includes a blocking condenser 63tapped on coil L2 and the directcurrent circuit includes resistance B3.In Fig. 6, the anode 40 of V2 is tapped to a point on L2 which isconnected to the platesource.

In Fig. 1, the coil L2 is paralleled by variable condensers C2 and C3connected through coupling and blocking condensers 46. The connectionbetween condensers C2 and I6 is grounded directly thereby includingcondenser C2 and crystal X in the grid circuit of the oscillator VI. Thecondensers C2 and C3 and inductance L2 form a tank circuit for themodulator tube V2. The anode 40 is connected substantially at one end ofthe said tank circuit, while one terminal of crystal X is connected to apoint intermediate the ends thereof.

Fig. 2 is quite similar to Fig. 1, the main differ ence being that inFig. 2,an inductance L3 replaces condenser C3 of Fig. 1 so that the tankcircuit in Fig. 2 comprises condenser C2 and in-- of C2 of Fig. 1.

In Fig. 4, the coil L2 is paralleled by variable condensers C2 and C3and the connection between these condensers is grounded directly. Thecondensers C2 and C3 are preferably operated'by one control shaft. Notein this figure that the anode 40 of modulator tube'1V2 is connected nearone end of tank circuit L2, C2, C3, and the electrode of crystal X isconnected to the other. end of the coil L2. The other electrode ofcrystal X is con? nected or coupled to the grid GI 01' tube VI so thatit is in a series grid circuit including condenser C3 to ground, etc. I

In 5, the modulator tank circuit comprises L2, C2, and the anode an oftube V2 iscoupled "by condenser 63 to a point on L2. One terminal ofcrystal X is grounded and one terminal of C2 aaeacsa figures it is inseries with one or more elements of the tank circuit connected withanode 43. The anode 40 is tapped through condenser 63 to a point on thetank circuit coil and the crystal has one electrode coupled to the highend of the tank circuit.

In Fig. 6, C2 .and C3 are in parallel while the crystal X is connectedin series with C3 so that it is included in the grid circuit ofoscillator VI. One terminal of crystal x is connected to one end ofinductance L2 of the tank circuit so that, as in all figures, anexcitation voltage from the modulator tank circuit is fed to thecrystal.

'As stated above, Fig. 7 will be described separately in detailhereinafter. The general description and comparison of Figs. 1 to 6 willnow be followed by detailed description of each of Figs. 1 to 6 and theoperation thereof.

In Figs. 1', 2, 3, 4, 5 and 6, the operation of the crystal oscillatorcircuit with tube VI is the same. The operation of grounded anodecrystal oscillator circuits has been described in prior art and need notbe described here.

The operation of the modulators in these circuits requires either theuse of a phase shifting network P in the grid excitation circuit of tubeV2 or the slight detuning of the modulator tank circuit L2, C2, etc.. toobtain the phase shift in the modulating radio-frequency excitationenergy to be delivered to tube VI. When the phase shifter at P is used,the modulator tank circuit comprising inductance L2, L3 and condenser C2and C3 is either detuned a large amount or it is substantially tuned.

In Fig. 1, assume the phase shifter P is in the grid excitation circuitof tube V2. The tank circuit L2, 02, C3 is detuned substantially fromresonance at the frequency of the excitation voltage.- Excitation energyis taken gem resistor RI and after passing through shifter P it isdelivered to the control grid 24 of tube V2. This excitation energy isof the frequency of operation of the oscillator of tube VI. In passingthrough the phase shifting network 1? the phase of the excitation energyis rotated say about degrees lagging (it could be 90 degrees or anyother angle in the leading direction dependin on the type of phaseshifter P used). In passing through the tube V2 this energy is amplifiedand reversed substantially in phase. Since the anode l0 and crystal Xare both connected to'the tank circuit on the same side of the radiofrequency ground which is at one end of the tank circuit, no furtherdisplacement of the phase of the wave energy takes place here. Thisenergy is delivered to the crystal X with a lagging phase angle of about90 degrees. Now the oscillating current from the grid of tube VI passesthrough the crystal and-through condenser C2 to ground. Since condenserC2 carries both the crystal oscillating current and the modulator tankoscillating current, this is thejelement which couples the modulatortank ci tiuit to the crystal oscillator circuit. The modulator currentin is grounded for" radio frequency through condenser 33 The crystal X,which is in the grid circuit of the oscillator tube VI is in'parallelwith condenser C2 in this figure whereas in the prior condenser C2 andthe crystal current in condenser C2 provide the two controlling voltagecomponents. These two voltage components are .adjusted to a phasedisplaced relation when no ner.

components. If the amplitudes of the currents and voltages in themodulator tube V2 are modulated by the signal oscillations from sourceA. the amplitude of the modulator excitation energy component deliveredfrom the modulator circuit through the crystal X to the control grid ofthe oscillator tube VI will also be modulated .in accordance .with thes'gnal. This causes corresponding changes in the frequency of operationof the oscillator. Thus, we have two components displaced in phase oneof which components is modulated in amplitude at signal frequency. As aconsequent, the resulting excitation on grid GI changes in phase atsignal frequency. There once. Consequently, during the process ofmodulation, the variation of anode current in eifect causes some changein the tuning of the modulator tank circuit. Therefore, the modulationexcitation energy component, which we assumed to have a 90 degree phaserelation to the oscillation energy component at the grid GI, not only ismodulated in amplitude but also is varied in phase angle a few degreeseach side of the average assumed 90 degree relationship during themodulation cycle. As seen from the vector rela tions, this causes theamplitude and the phase angle of the resultant grid excitation energy ofthe oscillator to vary according to the signal oscillations. These phasechanges are added to each cycle of the oscillations and, consequently,causes a change in the frequency of the oscillations. For any value ofmodulator bias within the operating range, the frequency shift or changeis limited to a ,value. whe'rfe an equal and opposite excitation'energy.phase shift oc-- for the grid cl of oscillator tube VI is the resultantof these two components. This resultant varies in phase due to amplitudeand/or phase variations of the component fed back by tube V2 of VI thephase displaced excitation voltages as described hereinbefore, bysl'mhtly detuning the modulator tank circuit L2, C2, etc. Under thesecircumstances the phase shifting element is not included in theconnection between the grid GI and grid 2i. when the phase shift isobtained by slightly. detuning the tank circuit, the excitation from themodulator, that is the voltage supplied from RI to 26; is amplified inV2, and sup plied to crystal X from the modulator in phase displacedrelation with respect .to the voltage of the oscillator VI due to thephase shift occasioned by detuning the tank circuit L2, C2, etc.

In my United States application Serial No.

338,838 filed June 5, 1930, I have illustrated several modificationswherein the piezo-electric crystal is effectively in series with thegrid and cathode of the tube VI as illustrated in Figs. 1 to 4 and 6 ofthe present application. In. my

said other application, Serial No. 338,837 filed June5, 1940, thecrystalis effectively shunted by "the impedance between the grid GI andthe curs whichis'caused by the opposite tuning of There is also a limitto the possible total range of modulation caused by limitations inmodulator tube current and by the angle of the excitation energycomponents.

Now assume that the phase shifter P is removed from the excitationcircuit for grid 24 of the modulator tube V2 and that the phasedisplaced relation between the crystal excitation voltage for grid GIand the voltage fed back from V2 is to be obtained in a difierent man-The tank circuit L2, 03, C2 isslightly detlmed from resonance preferablyon the low capacity side of resonance. Then the tank circuits representareactance. In this case the phase of the excitation energy on thecontrol grid 2| of modulator tube V2 is the same as that on the controlgrid GI of the oscillator tube VI.

in passing through the modulator tube V: this energy is amplified andreversed substantially (180 degrees) in phase. If the modulator tankcircuit was in tune it would represent a. resistance but since themodulator tank circuit is slightly detuned, it represents a reactanceand gives the excitation energy an additional phase rotation of sayabout 90 degrees in a direction depending on the direction of detuning.When detuned on the low capacity side the reactance is inductive.Consequently, the component fed from the modulator to the oscillator andthe oscillator components of the grid excitation are at approxiniately90 degrees phase displacement and the cathode of'tube VI, as illustratedin Fig. 5 of this application.

Moreover, I found during operation that a small air gap in the crystalholder gives a greater range of frequency modulation than a'wide air gapdoes.

The amplitude modulation incidentally caused by the frequency modulatoris nearly all suppressed bythe following stage and thatlittle whichremains can be eliminated in a radio transmitter by the use ofsubsequent amplitude limiter stages such as the usual frequencymultiplier stages. The frequency modulated carrier energy may then beamplified and/or multiplied in fredenser arrangement. It may be acombination ment of the modulator circuit may be accomcontrol gridradio-frequency excitation voltage connected pllshed by connecting R2 tosource 52.

It may be noted that a condenser C0 ha s been between the cathode oftube VI and ground. This capacity seems aid and strengthen theoscillator oscillations.

The modification of Fig. 2 is similar to the modification of Fig. 1described in detail above and a detailed description of the modificationof Fig. 2 and its operation is believed undesirable. It is noted,however, that in Fig. 2 the inductance L2 is common to the alternatingcurrent tank circuit and the oscillator grid and crystal circuit andforms the coupling means between the circuits. In Figs. 2 and 3, thecoils L2 and L3 may have mutual magnetic coupling between these coils.

In Fig. 3, which is in many respects like Fig. 2,

the positions of L2, C2 and L2 have been altered in the tank circuitwith respect to their positions in Fig. 2. In this modification L2 is inseries with the crystal in the oscillator grid circult and also in themodulator tank circuit.

In Fig. 4, C3 is the reactance common to the oscillator grid circuit andthe modulator tank circuit. y y

In Figs. 3 and 4, it should be noted that the anode II is connected to apoint on the tank cir-- cuit at one side of the radio-frequency groundconnection, while the crystal X is connected to a point on the tankcircuit at the other side of the radio-frequency ground connection. Thiscauses a phase reversal of the amplified energy to take place in thetank circuit. That is, the radio-frequency voltage at the crystal issubstantially reversed with respect to the radio-frequency voltage atthe anode II due to the said connections. In Figs. 1 and 2, no suchreversal took place. Now if a phase displacing means is used at P, thephase reversal is added to the phase displacement produced in P and thephase of the amplified voltage at the crystal leads or lags the phase ofthe generated voltage at that point depending on the direction of 'phasedisplacement at P.

When the phase shifter at P is omitted from the arrangements at Figs.'3and 4, the tank circuit is detuned andthe same phase reversals I takeplace supplemented by a phase shift de-- pending on the direction ofdetuning of the tank circuit.

The frequency modulator circuit of Fig. 5 contween the control grid oftube VI and ground.

The resistor RI isconnected between the control grid and ground. Thecathode of tube VI is connected part way up on resistor RI. Themodulator circuit. consists of signal source A, transformer T, tube V2and tuned circuit C2, L2.

. The cathodeof tube V2 is grounded. The screen grid 34 of tube V2 isgrounded for radio fre-.

quency by condenser 38. Signal source A is connected to the control grid24 of tube V2 through transformer T and resistor R2. Negative biaspotential is supplied to this grid from source 8 through the secondaryof transformer T and through resistor R2. The lower end of resistor R2is grounded for radio frequency-with a bypass tapped on resistor RIthrough a blocking condenser 28. One end of tank circuit C2, L2 isconnected to the grid of tube VI through a blockins condenser 64. Theother end of the tank circuit 'is grounded for radio frequency bycondenser Theanode of tube V2 is tapped on coil L2 directly or through ablocking condenser 3 of the tank circuit is grounded directly. R3supplies the anode with positive potential.

In Fig. 6, which is similar in many respects to Fig. 5, the crystal Xconnected to ground througha condenser C3 and the tank circuit C2, L2isconnected in parallel across condenser CI.

The operation of the arrangement of Fig. 5

will now be given. As in the prior modification, V

the grid GI and cathode I8 of oscillator VI operate at relatively highradio-frequency potential while the anode G2 is at groundradio-frequency potential, and the grid is in phase with but operates ata higher potential than the cathode, the same as in most ground anodetypes of oscillators. The crystal X acts as a frequency stabilizer.

Now, before this circuit can operate as a frequency modulator, the tankcircuit C2, L2 must be slightly detuned from resonance, we will assumeon the low capacity side for this case. Also, assume proper steadypotentials applied to the frequency modulator circuit. The excitation onthe control grid of tube V2 is in phase with that on the control grid oftube VI- In passin through tube V2 this energy is amplified and reversedin phase. In passing through the tank circuit C2, L2, the modulatingexcitation energ lated in amplitude. This causes the phase anglecondenser 30. The control grid of tube V2 is is again given a phaseshift or rotation of say 9 degrees lagging. Consequently, the excitationon the control grid of tube VI is the resultant of two components(assumed in this case to be disposed at degrees angle) and the resultantexcitation leads the normal oscillator excitation. If tank circuit C2,L2 had been detuned on the high capacity side, the modulating excitationenergy would be given a phase rotation of say 90 degrees in the leadingdirection so that the resultant excitation on the control grid of tubeVI would lag the usual phase of the oscillator excitation. Now, if tubeV2 is modulated in amplitude by the signal oscillations from source Athrough transformer T, the amount of excitation reaching the controlgrid of tube VI from the modulator circuit V2, C2, L2 will also bemoduof the resultant excitation on the grid of tube VI to change andvary in accordance with the signal. Consequently, the oscillatorfrequency is caused to vary in accordance with the signal. The outputfrequency modulated carrier will contain some amplitude modulation butthis can be eliminated by the use of ordinary limiter stages in thetransmitter. As a matter of fact, practically all frequency multiplierstages of a transmitter act'as fairly efiicient amplitude limiters. Whenanode current is blocked or! in the modulator tube V2 the crystaloscillator functions normally. The frequency is then slightly differentfrom that when modulator current flows.

A modification, Fig. 5, is shown in Fig. 6. In Fig. 6, the modulatingexcitation energy from tube V2 is applied to the grid of tube VI inseries with the crystal X (across the condenser C3) instead of beingapplied in parallel with the crystal X as in Fig. 5. Condenser C3 can beomitted altogether, if desirable. The general theory of the operation ofFig. 6 is about the same as for Figs. 1, 2 and 5 and will be fullyunderstood by aaeaase those versed in the prior art from the foregoingdescription. l

The frequency modulator circuit of Fig. 7 consists of a crystaloscillator circuit and a modulator circuit as in the prior arrangements.The crystal oscillator circuit consists of a tub VI, tank circuit C6, L8connected with the anode I0, crystal X, grid resistor RI andfeedbackcondenser C8. The cathode of Vi is grounded. The crystal X isconnected between the control grid GI and the cathode I8 of tube VI. Thegrid leak RI parallels the crystal X. The feedback condenser C8 isconnected between the anode II] and the control grid G! of tube V3. Thecenter point of tank coil L6 is grounded by condenser 58 for radiofrequency and one end of the tank circuit L5, C6 is connected to theanode Ill of tube VI. The modulator consists of tube V2, the phaseshifter element LP, the grid resistor R2 and the anode resistor R3. Thecathode of tube V2 is grounded and the grid resistor R2 is connectedbetween the control grid 24 and cathode 20 of tube V2. The anode 40 oftube V2 is connected to a source of positive potential through thecenter point of coil L6, and through resistor R3. The anode d2 of tubeV2 is also coupled to the grid of tube VI through a blocking condenser26' and through a phase shifting element coil LP. The grid 24 of tube V2is coupled to the tank coil LB through a blocking condenser 30 as shown.The screen grids-G2 and 34 of tube VI and V2 are modulated in phaseopposition from signal source A through transformer T.

Assuming proper steady potentials applied the circuitof Fig. 7, thecrystal oscillator circuit will oscillate by virtue of the feedback tothe control grid from the anode of tube VI through condenser C8 and thepiezo-electric action of the causes the frequency of the oscillator tovary. Since the modulating transformer circuit T modulates tubes VI andV2 in opposite sense a greater degree of phase change in the resultingradiofrequency voltage on the grid of VI is produced and, consequently,a greater degree of frequency modulation is obtained. If .the ground andbias tapping point on the secondary of transformer T isproperly chosenany amplitude modulation in the output is balanced out, leaving onlywave energy modulated in frequency in accordance with the signaloscillations from source A.

What is claimed is:

1. In a wave length modulation system, an oscillation generator of theelectron discharge tube type having electrodes including a control grid,a cathode, and an electrode serving as an anode, connected in anoscillation generating circuit including a piezo-electric crystal and areactance in series between said control grid and cathode, saidtubehaving other electrodes connected in an output circuit, anadditional electron discharge tube having an anode and having a controlelectrode coupled to said oscillation generating circuit to deriveexcitation voltages of the generated frequency therefrom, a tank circuitincluding a second reactance the first mentioned of said reactances anda third reactance, saidvthird reactance being connected in series withsaid first reactance, said series arrangement of said first and thirdreactances being connected in shunt to said second reactance, a couplingbe- 7 tween one end of said second reactance and crystal X, grid biasbeing by the rectified grid current passing through resistor RI. Thecrystal tends to hold constant frequency for the oscillator. The theoryof piezo-electric oscillators for regulating radio frequencies has beendescribed in prior art. The control grid 24 of tube V2 is excited fromtank circuit C6, L6 as shown. This energy is amplified in the anode oftube V2 and fed back to the grid of VI by way of phase shifter LP. If weassume that the grid of V2 is tapped on the upper end of tank circuitC6, L6, LI its excitation will be in phase with the grid of VI. Passingthrough tube V2 reverses themodulation excitation energy, i. e.,(0-180). Passing through phase shifter LP causes the modulationexcitation energy from tube V2 to lag say 90 degrees. The resultantexcitation on the grid of VI swings between the limits of zero to 90degrees leading in the case assumed. If the grid of V2 is tapped on theanode end of tank circuit C6, LE its excitation will be opposite to thaton the grid of VI because of the reversal in the tube VI.' Then thereversal in tube V2 brings the amplified modulating excitation back inphase with the voltage on grid GI of tube VI. However, the phase shifterLP again'causes a lag of 90 degrees, and the resultant excitationground, a coupling between the anode of said second named tube and apoint above ground radio frequency potential on one reactance, saidcouplings serving to feed amplified voltages to said generating circuitin phase displaced relation relative to the voltages generated therein,and means to control the gain of said second tube at signal frequency tothereby control the amplification of the voltages fed to said generatingcircuit and frequency modulate the oscillations generated.

2. In a wave length modulation system, an oscillation generator-of theelectron discharge tube type having electrodes including a control grid,a cathode, and an electrode serving as an anode, connected in anoscillation generating circuit including a piezo-electric crystal and areactance in series between said control grid and cathode,

said tube having other electrodes connected in an output circuit, anadditional electron discharge is lagging, which is opposite to the firstcase cited.

The excitation on the grid GI oftube'V I will, in either case, be theresultant of the oscillator VI excitation and the modulating excitationfrom the tube V2. Now, if the tube V2 is modulated in amplitude theamount of modulating excitation delivered from V2 to the grid of VI willalso be modulated in amplitude. If the oscillator delivers steadyexcitation, the result is a varying excitation phase angle on the gridof VI.

tube having an anode and having a control electrode coupled to saidoscillation generating circuit to derive excitation voltages of thegenerated frequency therefrom, a. tank circuit including an inductancethe first mentioned reactance and a second reactance, said secondreactance being connected in series with said first reactance, saidseries arrangement of first and second reactances being connected inshunt to said inductance, a coupling between one end of said inductanceand ground, a coupling between the anode of said second named tube and apoint on said inductance, said couplings serving to feed amplifiedvoltages to said generating circuit in phase displaced relation relativeto the voltages generated therein, .and means to control the gain ofsaid second tube at signal frequency to frequency modulate theoscillations generated.

3. In a wave length modulation system, an os cillation generator oftheelectron discharge tube type having electrodes including a controlgrid,. a cathode, and an electrode serving as an anode, v

connected in an oscillation generating circuit including apiezo-electric crystal and a capacity in series between said controlgrid and cathode, said tube having other electrodes connected in anoutput circuit, an additional electron discharge tube having an anodeand having a control electrode coupled to said oscillation generatingcircuit to derive excitation voltages therefrom, a tank circuitincluding an inductance said first mentioned capacity and a secondcapacity, said second capacity being connected in series with said firstcapacity, said series arrangement of first and second, capacities beingconnected in shunt to said inductance, a coupling between one end ofsaid inductance and ground, a coupling between the anode of said secondnamed tube and a point adjacent the other end of said inductance, saidcouplings serving to feed amplified voltages to said generating circuitin phase displaced relation relative to the voltages generated therein,and means to control the gain of said second tube .at signal frequencyto frequency modulate the oscillations generated.

4. In a wave length modulation system, an oscillation generator of theelectron discharge tube type having electrodes including a control grid,a cathode, and an electrode serving as an anode, connected in anoscillation generating circuit including a piezo-electric crystal and areactance in series between said control grid and cathode, said tubehaving other electrodes connected in an output circuit, an additionalelectron discharge tube having an anode and having a control electrodecoupled to said oscillation generating circuit to derive excitationvoltages therefrom,a tank circuit including a capacity said firstmensecond reactance being connected in series with said first reactance,said series arrangement of said first and second resctances beingconnected in shunt to said capacity, coupling between one end of saidcapacity and ground, a coupling between the anode of said second namedtube and a point on said third reactance, said couplings serving to feedamplified voltages to said generating circuit in phase displacedrelation relative to the voltages generated therein, and means tocontrol the gain of said second'tube at signal frequency to frequencymodulate the oscillations generated.

5. In a wave length modulation system,.an oscillation generator of theelectron discharge tube type having electrodes including a control grid,a cathode, and an electrode serving as an anode,

connected in an oscillation generating circuit including apiesc-eiectric crystal and an inductance in series between said controlgrid and cathode, said tube having other electrodes connected in anoutput circuit, an additional electron discharge tube having an anodeand having a control electrode coupled to said oscillation generatingcircuit to derive excitation voltages of the generated fre-- quencytherefrom, a tank circuit including as capacity said first mentionedinductance and a second inductance, said second inductance being'connected in series with said first inductance, said series arrangementof said first and second inductances being connected in shunt to saidcapaci y, a coupling between a terminal of said first inductance andground, a coupling between the anode of said second named tube and saidsecond inductance, said couplings serving to feed amplified voltages tosaid generating circuit in phase displaced relation relative to thevoltages generated therein, and means to control the gain of said secondtube at signal frequency to thereby control the amplification of thevoltages fed to said generating circuit and frequency modulate theoscillations generated.

6. In a wave length modulation system, an cecillation generator of theelectron discharge tube type having electrodes including a control grid,

' a cathode, and an electrode serving as an anode,

tioned reactance and a second reactance, said connected in anoscillation generating circuit including a piezo-electric crystal and areactancein series between said control grid and cathode, said tubehaving other electrodes connected in an output circuit, an additionalelectron discharge tube havingan anode and having a control electrodecoupled to said oscillation generating circultto derive excitationvoltages therefrom, a tank circuit including a second reactance saidfirst mentioned reactance and a third reactance, said third reactancebeing connected in series with said first reactance, said seriesarrangement of said first and third reactances being connected in shuntto said second reactance, a coupling between one end of said secondreactance and ground, acoupling between the anode of said second namedtube anda point adjacent said one end of said second reactance, saidcouplings serving to feed amplified voltages to said generating circuitin phase displaced relation relative to the voltages generated therein,and means to control the gain of said second tube at signal frequency tofrequency modulate the oscillations generated;

GEORGE L. UBBELMAN.

